The present invention relates to a method of determining the air density of intake air for an automobile engine.
When an automobile runs in a high altitude area, there occur drops in power, hard startings of engine, misfirings, knockings and the like because of a lower density of intake air, namely a smaller mass airflow admitted into the engine, than when it runs in a sea level area.
Commonly, in order to prevent these troubles in engines, countermeasures are taken so as to correct control parameters of engines according to the altitude where vehicles are operated by means of an equipment such as an atmospheric pressure sensor.
In a recent example of a technology not employing the costly atmospheric pressure sensor, as disclosed in Japanese Patent Application Laid Open No. 1991-185250, there is a proposed technique that an altitude judgment is made based on an air amount ratio between an actual air amount and a desired air amount which is calculated from the standard air amount data predetermined according to the throttle opening angle or the standard air amount data predetermined according to both the throttle opening angle and the engine speed.
However, the abovementioned technique using an air amount ratio between an actual air amount and a desired air amount has generally a tendency that the change of an actual intake air amount is too slow to respond to the change of the throttle opening angle when a parameter representing an air density is calculated based on the intake air amount admitted into the engine at the transient operating condition and consequently the actual amount of air corresponding to the throttle opening angle is read to be too small. As a result of this, the parameter representing an air density becomes smaller than an actual air density, and therefore not only an engine control based on the altitude judgment but also an engine control based on the air density come out of a proper control range due to erroneous air density data. That is to say, in this technique using the air amount ratio, it becomes difficult to measure a correct air density even under the same atmospheric pressure, when the vehicle runs at the rapidly changing operational condition.